Generally, in a screw compressor, lubricating oil is supplied to bearings supporting rotors and oil is injected into the compression cavities formed by the rotors and rotor casing to aid sealing the gap between the rotors and the gap between the rotors and the casing, and also to provide cooling sink for the gas charge in order to increase volumetric and thermal efficiencies.
Such a screw compressor requires a large amount of lubricating oil for lubricating the bearings and shaft seal element and for lubricating the rotors and cooling the gas charge. When operating gas dissolving type refrigerating machine oil is used as lubricating oil, operating gas dissolved in the oil is flash-evaporated from the oil in the compression cavities, which induces early pressure rise in the compression cavities resulting in increased leak of the gas charge toward the suction side and decreased volumetric efficiency. Conventionally, it has been thought effective to decrease amounts of gas charge, i.e., operating gas dissolved in the oil as far as possible in order to minimize the influence as mentioned above.
Dissolution characteristic of operating gas into lubricating oil is such that, the higher the pressure and the lower the temperature of oil, the larger the amount of operating gas dissolved into the oil. Therefore, it is thought effective to increase discharge temperature of the gas charge in order to decrease dissolved amounts of operating gas, and various devisal has been made.
However, when discharge temperature is raised too much, scuffing of rotors occurs due to thermal expansion of the rotors lubrication of the bearings and shaft seal elements becomes insufficient due to heat transferred to them from the rotor casing. Therefore, elimination of the affection of flash-evaporation of dissolved operating gas has not been achieved sufficiently by increasing discharge temperature. Particularly, in the case of two-stage screw compressor, when high pressure oil dissolving a large amount of operating gas is supplied to the bearings, shaft seal element, and compression cavities of the low-pressure stage compressor, proportion in weight of operating gas flash-evaporated from lubricating oil relative to operating gas sucked in the compressor increases due to lower pressure in the compression cavities of the low-pressure stage compressor, and compression efficiency of the compressor decreases.
In the case of a conventional two-stage screw compressor, lubricating oil supplied to the bearings and shaft seal element of the low-pressure stage compressor is supplied to the compression cavities of the low-pressure stage compressor, the operating gas compressed by the low-pressure stage compressor is sent together with the oil containing dissolved operating gas to the compression cavities of high-pressure stage compressor to be compressed and discharged from the high pressure stage compressor.
FIG. 5 is a longitudinal sectional view of the conventional two-stage screw compressor mentioned above. In FIG. 5, reference numeral 01 is a casing in which main components of the compressor are housed, 02 is a low-pressure stage compressor comprising a male rotor and a female rotor of low-pressure stage, 03 is a high-pressure stage compressor comprising a male rotor and a female rotor of high-pressure stage for further compressing gas compressed in the low-pressure stage compressor. Reference numeral 04 is a common rotor shaft of the male rotor and driven by a drive device not shown in the drawing.
Reference numeral 05 is a mechanical seal, and 06, 07, and 08 are bearings supporting for rotation of the rotor shaft 04 at the inlet side of the low-pressure stage compressor, at the intermediate section between the lower and high-pressure stage compressor, and at the inlet side of the high-pressure stage compressor, respectively. A common female rotor shaft not shown in the drawing is supported by bearings in the same way. Reference numeral 011 is an oil supply port through which lubricating oil h separated from the compressed operating gas discharged from the high pressure stage compressor in an oil separator not shown in the drawing and containing dissolved operating gas is supplied to the mechanical seal 05 and bearings 06, 07 via an oil passage 012. The oil after lubricated the mechanical seal and bearings is injected into the compression cavities of the low-pressure stage compressor 02 through an oil supply hole 021.
On the other hand, lubricating oil h containing dissolved refrigerant is supplied from the oil separator through an oil supply port 014 to the bearing 08 via an oil passage 015, then injected into the compression cavities of the high-pressure stage compressor 03 through an oil supply hole 017. Reference numeral 018 indicates an inlet port for sucking operating gas r into the low-pressure stage compressor 02. Operating gas compressed in the low-pressure stage compressor 02 is introduced to the high-pressure stage compressor 03 via a gas passage 019, further compressed therein, and discharged from a discharge port 020.
Operating gas flash-evaporated from lubricating oil supplied to the compression cavities of the low-pressure stage compressor affects to reduce volumetric efficiency of the lower and high-pressure stage compressor. Particularly, in the case of two-stage compressor, flow rate of operating gas depends on volumetric efficiency of the low-pressure stage compressor, so influence of supplying lubricating oil containing dissolved operating gas to the compression cavities of the low-pressure stage compressor is significant.
Amounts of operating gas released from lubrication oil increases with decreasing pressure, so operating gas released from lubricating oil significantly affects the volumetric efficiency of the low-pressure stage compressor, and as a result operating gas flow of the two-stage compressor is significantly reduced.
In patent literature 1 is disclosed a refrigerating cycle in which the two-stage screw compressor is composed such that lubricating oil supplied to the low-pressure stage compressor from the oil separator provided in the downstream side from the high-pressure stage compressor is introduced to the intermediate casing of the two-stage compressor, thereby preventing reduction of refrigerating capacity.    Patent literature 1: Japanese Patent No. 3653330